Various types of devices are known in the art for separating and removing solid particulate from fluid flows. Such devices include settling chambers in which large solid particles settle by gravity; centrifugal collectors (cyclones) and inertial dust separators which snake use of inertial effects arising from changes in the direction of the fluid flow to separate particulate matter; filters (baghouses) in which dust laden fluid passes through cloth, paper, fiberglass, etc.; electrostatic precipitators in which the particles are electrically charged in a high voltage electric field and then are drawn to an oppositely charged electrode; and other devices, such as wet scrubbers, in which the dust particles are brought in contact with a liquid and subsequently carried away.
As discussed in our commonly assigned U.S. Pat. No. 5,221,305, which is hereby incorporated by reference, each of the above type devices have characteristic operational parameters which are in part dictated by the device's principle of operation, and which in turn make them better suited to certain applications. One such parameter is optimum flow velocity. For example, electrostatic precipitators must operate at a relatively low gas velocity seldom exceeding 2 meters per second (2 m/s), while inertial dust separators operate at velocities typically between 15 and 30 m/s or even higher. For many industrial applications, the higher velocity is desirable since it implies a higher throughput (the product of a fluid flow velocity and the cross-sectional area of the flow through the device). Therefore, for a given required throughput, the higher the allowed gas velocity, the smaller can be the size of the device. On the other hand, higher velocity implies higher pressure drop across the device, hence higher costs of energy required to operate the device. This creates an tradeoff between throughput and energy requirements.
Another parameter is the fractional dust separation efficiency of a device, commonly defined as the ratio of the weight of dust separated to the weight of incoming dust, commonly expressed in percent (%) as a function of the particle size (diameter) D. The fractional efficiency of a given device is usually presented in the form of a histogram or curve on which the values of efficiency for narrow intervals of particle sizes are plotted as a function of an average particle size in each interval. W. Strauss, "Industrial Gas Cleaning" Pergamon Press, 1966.
Existing inertial separators will usually show a high (over 80% to 90%) efficiency of separation of particles within a certain range of diameters which typically includes the larger .particles. The efficiency begins to decline for smaller sized particles. For example, typical industrial cyclones have high efficiency for separation particle sizes greater than 20 to 30 micrometers.
Such high efficiency, at a relatively high flow velocity, in combination with inherently simple operation and low maintenance, make inertial type separators relatively attractive options for use in many industrial and commercial applications. However, the steep decline in particle separation efficiencies for smaller particle sizes, is a drawback of such importance as to lead to the use of other devices such as electrostatic precipitators or baghouses in conjunction with inertial separator.
In one example of a basic inertial type separator, dust-laden fluid flow (e.g., gas and particles exhausted as a result of combustion) is intercepted by a system of spaced inclined plates, or louvers. The gas flow is deflected from its original direction by the louvers whereby part of the gas flow proceeds to pass between the plates or louvers and into the space behind them, while the dust particles continue to move by inertia and collect in the small remaining part of the gas flow, from which they are usually removed with dust-separators of a different kind, such as a filter. The cleaning efficiency depends, among other factors, on the angle of inclination and distances between the plates. A drawback of such a device is that the efficiency for removing smaller particles is relatively low and the use of a different type of device to remove the particles increases the system's complexity and cost.
Another prior art dispersed phase separator is disclosed in U.S. Pat. No. 3,342,024 which, in order to increase the cleaning efficiency in the separation of the dispersed phase, employed two systems of spaced elements, or two gratings, positioned sequentially along the flow of the media to be cleaned. A disadvantage of this device however, is that the two gratings are positioned so close to each other that the flow entering the second grating may be turbulent thereby resulting in reduction of the cleaning efficiency of the second grating.
An additional prior art aerodynamic device, disclosed in U.S. Pat. No. 5,221,305 includes a cylindrical channel having constant cross-section along the length of the device, with a grating comprised of identically convexly shaped elements (rings) axially spaced apart and transversely staggered or offset from a wall within the channel. The axial distance d between adjacent elements is constant and the innermost edge of each element is disposed closer by distance W to the channel axis than an innermost edge of the immediately upstream element. The outline of the outer edges of the plurality of the grating elements is disposed at an oblique angle with respect to the longitudinal axis of the channel. The device also contains means of removing the dispersed phase connected to the hopper and to a source of reduced pressure.
However, the absence of specific guidelines for choosing the optimum parameters of the device, such as the height and the width of the elements, a distance between them, and a transverse shift W, does not allow one to design a separator which will be characterized by filtering out particles in a specific range of diameters.
The first element (ring) of the device, which is connected in a fluid-tight manner to the casing, has the same cross-sectional shape as the rest of the elements. This feature decreases the useful input area of the device, as the outer parts of that first element do not provide for any useful function.